A magneto-inductive flow measuring device includes, usually, the following components:                a measuring tube, through which a medium flows essentially in the direction of the measuring tube axis;        a magnet arrangement, which produces an alternating magnetic field passing through the measuring tube essentially perpendicularly to the measuring tube axis;        two measuring electrodes positioned in the measuring tube on a connecting line directed essentially perpendicularly to the measuring tube axis and to the magnetic field; and        a control/evaluation unit, which determines volume- or mass-flow of the medium through the measuring tube on the basis of the measurement voltage tapped on the measuring electrodes.        
Magneto-inductive flow measuring devices make use of the principle of electrodynamic induction for measuring volumetric flow Charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction. This measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube. It is thus, proportional to volume flow rate. In the case of known density of the medium, it is, moreover, possible to derive also the mass flow rate of the medium flowing through the measuring tube. The measuring electrodes are usually galvanically or capacitively coupled with the medium.
Usually, control of the alternating magnetic field is accomplished using the coil current. In the ideal case, the magnetic field is proportional to the electrical current flowing in the coil arrangement, since the measuring, or field-frequency of the magneto-inductive flow measuring device (thus, the frequency, with which reversal of the alternating magnetic field occurs) is dependent, to a high degree, on the inductance of the coil arrangement. Due to manufacturing tolerances, the inductance of the coil arrangement is different from flow measuring device to flow measuring device. Moreover, there arise, during the reversal of the magnetic field, eddy currents in the pole shoes and in the cores of the coil arrangement. Due to the eddy currents, the coil current measured externally of the coil arrangement does not correspond to the sum of the current flowing in a coil and the current produced by the eddy currents. If the current measured externally of the coil arrangement is used as control variable, then, yes, the current is constant, but not the magnetic field. The actual rise time to reach a constant magnetic field is, as a result, a sensor-specific variable burdened with a relatively large measurement error.
The field frequency, or the measuring frequency, is fixed in the case of known magneto-inductive flow measuring devices. It is chosen so large, that even for the conceivably most unfavorable rise time, the measuring time for performing a measuring of volume- or mass-flow with the desired accuracy of measurement is sufficiently large. The disadvantage of this estimated solution is, however, clear: Since the field- or measuring-frequency is, in principle, sized greater than actually necessary, the number of measured values provided per unit of time is smaller than in the case of an optimally tuned measuring frequency. The end result of this limitation is reduced accuracy of measurement.